This invention pertains to the art of inverters or switching circuits and has specific application to high-current switching circuits using power transistors.
The invention is particularly applicable to switching circuits that use insulated gate bipolar transistors (IGBTs) and zero-voltage-switching (ZVS) capacitors and will be described with particular reference thereto. However, it will be appreciated that the invention has broader applications such as a safety circuit for protecting against open circuit voltage spikes, or snubber networks for high current devices to eliminate switching losses and protect electrical devices, circuitry or components from the voltage spikes. The invention thus may be advantageously employed in other environments and applications.
In industrial electronics and other applications, inverters or switching circuits are often used to convert direct current (DC) to alternating current (AC). Switching circuits are capable of supplying high current at high voltage at specific AC frequencies for applications such as induction heating.
One type of solid-state switching circuit contains power transistors that makes use of current gains greater than unity. Examples of such power transistors are insulated gate bipolar transistors (IGBTs) and metal-oxide-semiconductor-field-effect transistors (MOSFETs). When the power transistors are switched off, the turn-off times are usually intended to be very fast, and even when minimized, the stray inductance of the switching circuit can result in unacceptable voltage spikes in excess of the normal rating of the device, can potentially harm components in the circuit, and contribute to switching losses. As switching circuits must deal with higher and higher frequencies in high power industrial environments, this problem becomes more and more serious.
To control the voltage spikes and to reduce switching losses, ZVS capacitors are used in conjunction with the power transistors. In one example, a ZVS capacitor is wired in parallel to an IGBT. In addition, a clamping diode is typically wired in parallel to the ZVS capacitor and IGBT. However, a major problem with such a ZVS capacitor configuration is that the capacitor creates a resonant circuit with the bus inductance. Accordingly, when the IGBTs are switched and cause voltage spikes, severe oscillations (ringings) are set up in the circuit which can damage or cause malfunctions in the IGBTs. The RMS current requirements for the capacitor can be more than doubled.
Snubber circuits for damping circuit ringing comprise merely placing a resistor in series with the ZVS capacitor. FIG. 6 shows such a circuit. In this regard, a resistor may be placed in series with the capacitor, but at typical switching currents and speeds, the resistor will have to dissipate significant power.
The present invention contemplates a new and improved method and apparatus which simply and economically overcomes all of the above-referenced problems and others and provides a substantial improvement in efficiency in the handling of the switching.